It seems that there are things we watch live and things we watch when we want to. From the AP via the UPoR:

Meanwhile, Nielsen said NBC narrowly beat CBS to be the most-watched network last week. The three most popular programs on the air last week were all on NBC — two editions of “America’s Got Talent” and Sunday’s exhibition football game between the New York Giants and Buffalo Bills.

Wait a few days, however, and CBS is likely to claim the week’s crown. That’s because Nielsen’s weekly rankings are based on who watches the programs live or within the next 24 hours, and reality shows and sports programming are most likely to be watched live.

Dramas, which dominate CBS’ schedule, are more likely to be time-shifted. For example, the July 14 episode of CBS’ “Under the Dome” was seen by 7.6 million people within the first 24 hours, and just under 12 million within seven days — a 57 percent increase due to people watching on their DVRs or on demand. That week’s one episode of “America’s Got Talent” saw only a 17 percent bump when the full week’s viewership was taken into account.